The invention relates to a method of locating an interfering transmitter or xe2x80x9cjammerxe2x80x9d for a satellite telecommunications system.
Satellite telecommunications are particularly sensitive to interfering transmitters or jammers. When the interfering transmission reaches the satellite, it affects all of the communications covered by the earth satellite, which in general means coverage of a very large geographical extent. It is therefore particularly important for satellite telecommunications operators to be capable of detecting and locating interfering transmitters in order to take measures to have the jamming stopped. By way of example, such measures can be intervention with the local authorities from which the jammer depends so as to have them ensure that the undesired transmission ceases.
The invention provides particularly simple and effective means for detecting and locating interfering transmitters.
To locate jammers of a satellite telecommunications system, the invention makes use of a moving detection satellite, preferably in low or medium orbit, the satellite being dedicated solely to detection, and for location purposes, at least two of the three following measurements are performed:
a measurement of the difference between the instants of arrival at the telecommunications satellite and at the detection satellite of signals transmitted at the same time by the jammer,
a measurement of the difference between the frequencies received by the telecommunications satellite and by the detection satellite, for a signal transmitted by the jammer; and
an interferometric measurement of the angle between a direction associated with the detection satellite and the straight line between said satellite and the jammer.
The difference between the instants of arrival at the telecommunications satellite and at the detection satellite of signals transmitted at the same time by the jammer, can be used to deduce that the jammer lies on the surface of a hyperboloid whose foci are constituted by the two satellites (of known positions). The curve of intersection between the hyperboloid and the Earth can be calculated.
The measurement which consists in determining the difference between the frequencies received by the two satellites for a given jamming signal makes it possible to define an ellipsoid whose foci are the two satellites and on which the jammer is to be found. It is then possible to calculate the curve of intersection between said ellipsoid and the surface of the Earth. It is recalled at this point that because of the movements of the satellites and of the earth, the frequencies are offset by a Doppler effect which is proportional to the relative speed vector projected onto the satellite-jammer axis.
Thus, with two measurements it can be seen that the position of the jammer corresponds to a point of intersection between the curves of intersection between the Earth and the hyperboloid and the ellipsoid.
To lift ambiguities, it is also possible to repeat the first two measurements. Repeating the measurements at a different instant will necessarily give other curves of intersection since the detection satellite is a moving satellite.
To measure the difference between the arrival instants at the two satellites corresponding to transmission at the same instant by the jammer, and also to measure the difference between the received frequencies, conventional correlation methods are used.
The angle measurement defines a cone whose apex is constituted by the detection satellite and whose axis is said direction associated with the satellite. The jammer lies on the intersection between the cone and the earth.
Thus, in general, the position of the jammer is determined by the points of intersection between two curves of intersection, with the third being usable to lift ambiguities.
For a satellite telecommunications system, the invention provides adding a detection satellite which can be single and which can have a payload that is simple. In addition, the detection satellite can be used for locating the jammers interfering with a plurality of telecommunications satellites forming parts either of a single telecommunications system, or else of different systems. Thus, there is no need for each telecommunications operator to have its own detection satellite, since the satellite can be shared between a plurality of operators.
For measurements using the satellite of the telecommunications system, it is preferable to make use of the signals picked up by the ground station of the system. Under such conditions, the position-locating method does not require means to be installed on board the telecommunications satellite, and can thus be implemented even if no provision was made for such detection when the telecommunications system satellite was launched.
Thus, the invention provides a method of locating a jammer interfering with a satellite telecommunications system. To locate the jammer, at least two of the following three measurements are performed:
the difference is measured between the instants of arrival at the telecommunications satellite and at a satellite dedicated to detection of signals transmitted at the same time by the interfering transmitter;
the difference between the frequencies received by the telecommunications satellite and by the detection satellite is measured for a signal transmitted by the interfering transmitter; and
the angle between a direction associated with the detection satellite and the straight line between said satellite and the interfering transmitter is measured interferometrically.
In an implementation, the telecommunications system includes a ground station and measurements involving the satellite of the telecommunications system are performed on the basis of signals picked up by the ground station.
In an implementation, a time reference is given to the ground station, e.g. a reference of the GPS type.
In an implementation, the position of the jammer is located in a ground station.
In an implementation, the telecommunications satellite is of the geostationary type or of the geosynchronous type.
In an implementation, the detection satellite is used to detect jammers interfering with a plurality of telecommunications systems.
In an implementation, the detection satellite is a moving satellite in low or medium orbit.
The present invention also provides a method of locating jammers that can interfere with a satellite telecommunications system. A moving satellite is provided that is dedicated solely to detection and that is suitable for covering the region covered by the telecommunications system, and at least two of the following three measurements are performed:
a measurement of the difference between the instants of arrival at the telecommunications satellite and at the detection satellite of signals transmitted at the same time by the jammer,
a measurement of the difference between the frequencies received by the telecommunications satellite and by the detection satellite, for a signal transmitted by the jammer; and
an interferometric measurement of the angle between a direction associated with the detection satellite and the straight line between said satellite and the jammer.
In an implementation, there is a single detection satellite.
In an implementation, the detection satellite is put into orbit after the satellite telecommunications system has been put into place.